Scale adaptive simulation of transient behavior in a transonic centrifugal compressor with a vaned diffuser

Three-dimensional, compressible, unsteady Navier-Stokes equations are solved to investigate the unsteady flow behavior in a transonic centrifugal compressor. The computational model is a high compression ratio centrifugal compressor (4:1) consisted of an inlet duct, an impeller (15 main blades and 15 splitters) and a diffuser vane with 24 two-dimensional wedge vanes. The aim of this study is to conduct a comprehensive assessment of the ability of a hybrid scale-adaptive simulation (SAS) turbulent model to characterize the transient flow structures within the compressor passages. The main idea of SAS approach, an improved URANS (unsteady Reynold-averaged Navier-Stokes) model, is based on the introduction of von Karman length scale into the turbulent scale equation which results in LES-like behavior in unsteady regions of the flow field. A numerical sensitivity test is performed to validate the computational results in terms of pressure ratio and compressor efficiency. Instantaneous and mean flow field analyses are presented in the impeller and the vaned diffuser. Applying transient simulations, it is shown that the interaction between the pressure waves and the surface pressure of the diffuser blades leads to a pulsating behavior within the diffuser. Moreover, spectral analysis is evaluated to analyze the BPF tonal noise as the main noise source of centrifugal compressors. In addition, the current SAS results are compared with those of the URANSSST (shear stress transport) approach to show the ability of SAS approach in the prediction of the turbulent structures where the SAS model leads to a much better resolution of the unsteady fluctuations. This study shows that the current SAS approach, as an alternative to the existing hybrid RANS/LES methods, is promising in terms of prediction of transient phenomena like LES, but with a substantially reduced turn-around time.